Optical illusion and Special effect

Optical illusion noun - An optical phenomenon that results in a false or deceptive visual impression.

Special effect and optical illusion are semantically related. In some cases you can use "Special effect" instead a noun phrase "Optical illusion".

Special effect noun - An effect used to produce scenes that cannot be achieved by normal techniques (especially on film).

Optical illusion and special effect are semantically related. You can use "Optical illusion" instead a noun phrase "Special effect".